Lapping and polishing by conventional techniques of relatively large optical members such as are required for astronomical observations are very time-consuming because it is extremely difficult to achieve the desired shape with the required accuracy of fractions of the wavelength of light, typically about 10-50 nm RMS, over the total surface to be worked.
To shorten the work time, an apparatus has already been proposed wherein a tool covering the entire surface of the workpiece to be processed is provided in the shape of a flexible membrane. Moreover, the tool, on whose lower side the polishing elements are fastened, oscillates tangentially over the workpiece under a series of loading units. These loading units are stationary relative to the workpiece and produce a pressure distribution calculated from the deviations of the workpiece from the desired shape. If desired, these loading units can be moved together with their support laterally relative to the membrane by an amount which is small in comparison to the amplitude of the membrane movement. In this way, the loading units are prevented from impressing the workpiece which, for example, could occur if the stiffness of the membrane is selected as being relatively small.
This apparatus is disclosed in U.S. Pat. No. 4,606,151 which is incorporated by reference herein. With this apparatus it is difficult, nevertheless, to work on very large members such as telescope mirrors with a diameter of four meters or larger because the correspondingly large tool is then difficult to handle. Problems arise, among others, with respect to the metering of the polishing liquid which must always be supplied very uniformly as well as with the preparation of the tool, that is, applying the tool to the workpiece and the pressing of the tool to its proper shape between subsequent working cycles. In addition, large local pressure differences on the rearward side of the tool can cause running of the polishing means carrier, so that the tool deforms rather quickly. This leads to a reduction of the useful dynamics of the polishing process.
Furthermore, with the known apparatus, it is not possible without additional effort to work on grazing incidence optical devices such as conical shells of Wolter telescopes for the X-ray astronomy.
Another polishing apparatus which is similar to that discussed above is disclosed in U.S. Pat. No. 2,399,924. This apparatus also uses a flexible membrane as a tool which extends over the entire surface to be worked upon. This membrane is loaded according to a pressure distribution adapted to a predetermined material removal. With this apparatus, the workpiece to be worked upon is rotated at the same time.
However, with this kind of apparatus, it is only possible to polish away rotationally-symmetrical deviations from the desired shape of the workpiece. Furthermore, it is not possible to eliminate short periodic deviations because the pressure distribution on the rearward side of the tool shifts with the polishing movements relative to the workpiece, since the pressure distribution is produced by weights which rest on the membrane and move with the membrane over the surface to be worked upon.